What to look for depends a lot on your budget. And you seem to be looking for something that can make good orchestral sounds as well as synth sounds. These are both very different types of sounds. Orchestral is probably best with a rompler/sample based engine like a Yamaha Mo/Motif, Korg Triton or Roland Fantom. But all for these are in a very much higher price range than the Casio you have. Even the Nord that Justin mentioned which would be great for synth type sounds is not exactly cheap.

And it certainly sounds like you would be more interested in a rack mounted module you can MIDI up to your Privia so you don't have to drag 2 keyboards around with you. Is that right or were you looking for something with a keyboard so you could play both keyboards at the same time?